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L-arginine-dependent reactive nitrogen intermediates and the antimicrobial effect of activated human mononuclear
1Division of Infectious Diseases, Cornell University Medical College, New York, New York 10021.
Abstract:
The L-arginine-dependent generation of reactive nitrogen intermediates (RNI) has been identified as a key intracellular antimicrobial mechanism of activated mouse macrophages. To determine the role of this mechanism in the activity of human mononuclear phagocytes, monocyte-derived macrophages activated in vitro by interferon (IFN)-gamma and monocytes from patients receiving IFN-gamma as therapy were treated with NG-monomethyl-L-arginine (NMA) or arginase. Neither competitive inhibition of L-arginine metabolism (NMA) nor depletion of L-arginine (arginase) altered intracellular antimicrobial activity against Toxoplasma gondii, Chlamydia psittaci, or Leishmania donovani. In contrast, NMA and arginase readily reversed the antimicrobial effect of mouse peritoneal macrophages stimulated either in vitro or in vivo by IFN-gamma, and activated mouse but not human cells could be induced to release enhanced levels of nitrite. These results suggest that the L-arginine-dependent generation of RNI is a species-restricted macrophage mechanism unlikely to participate in the intracellular antimicrobial activity of IFN-gamma-stimulated human mononuclear phagocytes.
Insights
Reactive nitrogen intermediates (RNI) fight microbes in mouse macrophages but not in human cells. Interferon-gamma (IFN-gamma) activation of human macrophages doesn't rely on this L-arginine-dependent pathway.
Area of Science:
- Immunology
- Cell Biology
- Microbiology
Background:
- The L-arginine-dependent generation of reactive nitrogen intermediates (RNI) is a known antimicrobial mechanism in activated mouse macrophages.
- The role of RNI in human mononuclear phagocytes, particularly after interferon-gamma (IFN-gamma) activation, remains to be fully elucidated.
Purpose of the Study:
- To investigate the involvement of L-arginine-dependent RNI production in the antimicrobial activity of human mononuclear phagocytes activated by IFN-gamma.
- To compare this mechanism between human and mouse macrophage responses to IFN-gamma stimulation.
Main Methods:
- Human monocyte-derived macrophages and monocytes from IFN-gamma-treated patients were utilized.
- Cells were treated with NG-monomethyl-L-arginine (NMA) or arginase to inhibit L-arginine metabolism or deplete L-arginine.
- Antimicrobial activity against Toxoplasma gondii, Chlamydia psittaci, and Leishmania donovani was assessed.
- Nitrite release was measured in activated mouse and human cells.
Main Results:
- Inhibition or depletion of L-arginine did not affect the intracellular antimicrobial activity of human macrophages against tested pathogens.
- Conversely, NMA and arginase reversed the antimicrobial effect in IFN-gamma-stimulated mouse macrophages.
- Activated mouse macrophages, but not human cells, released enhanced nitrite levels upon IFN-gamma stimulation.
Conclusions:
- The L-arginine-dependent generation of RNI appears to be a species-restricted mechanism.
- This pathway is unlikely to be a significant contributor to the intracellular antimicrobial activity of IFN-gamma-stimulated human mononuclear phagocytes.